Summary

• Add docs/guides/STORAGE_BACKENDS.md covering the complete storage subsystem
• Document dual interface architecture (storage_backend + snapshot_storage_backend)
• Detail all 10 backend types across 4 categories: memory, file (JSON/binary/CSV), database (SQLite/PostgreSQL/MySQL), cloud (S3/GCS/Azure Blob)
• Include backend comparison matrix with decision guide
• Document configuration validation, retention policies, and capacity sizing
• Provide custom backend implementation guide with Redis example
• Include 4 configuration examples: tiered storage, development, PostgreSQL production, cloud archival

Resolves #457

Test Plan

• All 4 referenced source files exist and paths are correct
  • storage_backends.h, monitoring_core.h, result_types.h, error_codes.h ✓
• Backend types match actual storage_backend_type enum values
  • All 10 types documented: memory_buffer, file_json, file_binary, file_csv, database_sqlite, database_postgresql, database_mysql, cloud_s3, cloud_gcs, cloud_azure_blob ✓
• Interface methods match actual source signatures
  • snapshot_storage_backend: store, retrieve, retrieve_range, size, capacity, flush, clear, get_stats ✓
  • storage_backend: store, retrieve, retrieve_range, capacity, size, clear, flush ✓
• Document covers all 4 acceptance criteria from issue
  1. All backends documented with configuration (Section 4, all 10 types) ✓
  2. Comparison table with selection criteria (Section 3) ✓
  3. Retention and cleanup policies documented (Section 5) ✓
  4. Custom backend implementation guide (Section 6, Redis example) ✓
• Cross-references to other guides are valid
  • COLLECTOR_DEVELOPMENT.md, EXPORTER_DEVELOPMENT.md, ARCHITECTURE.md ✓